1. Field of the Invention
The present invention relates to a semiconductor device such as a transistor, and more particularly, to an improvement in a wire bonding structure for connecting a semiconductor chip and a lead terminal.
2. Description of the Prior Art
A conventional manner of bonding a transistor which is typical of a semiconductor device of this type is as shown in FIG. 1. Specifically, a chip 1 of the transistor device is die-bonded onto a mount portion 3 provided in the center of a lead frame 2, and bonding pads 4' and 5' respectively formed at the right and left sides on the obverse surface of the chip 1 and wide ends 6a and 7a of lead terminals 6 and 7 provided on both sides of the mount portion 3 are wire-bonded. With this bonding, a collector electrode formed on the reverse surface of the chip 1 is electrically connected to the mount portion 3, and a base electrode and an emitter electrode where the bonding pads 4' and 5' are provided are respectively connected to the lead terminals 6 and 7 by bonding wires 8.
In such a bonding structure, an aluminum wire which is inexpensive and highly practicable is widely used as the bonding wire 8. As the wire bonding method, a wedge bonding method is generally used.
Bonding by the wedge bonding method using the aluminum wire 8 is performed in a manner as shown in FIG. 7. First, in the first step, the wire 8 is supplied to the bonding pads 4' (5') from a wire feeding nozzle 9, and the tip of the wire 8 is fixed to the bonding pad 4' (5') by rubbing it by means of a head 10 by using supersonic waves. Then, in the second step, the head 10 is moved to the terminal end 6a along a predetermined line. Lastly, in the third step, an end portion 8a of the wire 8 at which the wire 8 is cut is rubbed by using supersonic waves to bond it to the terminal end 6a (7a).
Referring to FIG. 8a, there is shown a configuration of the wire end portion 8a connected to the bonding pad 4' formed on the surface of the chip 1. As shown in FIGS. 1 and 8a, the bonding pads 4' and 5' are formed to be longer along the sides of the chip 1, for example, to be rectangular, since it is necessary to rub the wires 8 on the pads 4' and 5' toward the front and rear of the figure. The wire end portions 8a fixed to the pads 4' and 5' are rubbed into a configuration extended toward both sides by receiving a rubbing force from the head 10.
It is necessary that the line along which the head 10 is moved in the second step be largely bent on the way. This is because the right and left lead terminals 6 and 7 are further displaced rightward and leftward by a predetermined amount relative to the right and left bonding pads 4' and 5' since a lead terminal 3a extending from the mount portion 3 is inevitably located between the lead terminals 6 and 7. Therefore, with this bonding structure, the wires 8 connecting the bonding pads 4' and 5' and the terminal ends 6a and 7a are largely bent on the way. Accordingly, the time required for the bonding process increases, so that the quantity of production per unit time is largely restricted.
Further, with this conventional bonding structure, when it is employed for a transistor, it is difficult to arrange an emitter region formed around the bonding pads 4' and 5', so as to form a symmetrical pattern, since the bonding pads 4' and 5' serving as the base electrode and the emitter electrode, respectively, are provided in parallel on the chip surface. For this reason, obtaining a required performance as a transistor and achieving a reduction in size of the chip are hardly compatible with each other.
To solve the above-mentioned problem, the head 10 may be moved along straight lines connecting the bonding pads 4' and 5' and the terminal ends 6a and 7a. That is, the head 10 is linearly moved while being inclined at a necessary angle to the sides of the chip 1. In this case, however, since the wire end portions 8a are fixed to the bonding pads 4' and 5' obliquely to the rectangular pads 4' and 5' as shown in FIG. 8b, it is necessary to form the pads 4' and 5' to be wider in order that the wire end portions 8a rubbed by the head 10 into a configuration extended toward both sides do not protrude from the pads 4' and 5'.
However, for example in the above-described transistor, if the bonding pads 4' and 5' are wider, the effective emitter active region formed around the bonding pads 4' and 5' does not reach the central portions of the pad regions, and accordingly, it is necessary to increase the size of the chip 1. As a result, the cost increases.